With the recent development of optics-applied technique, shorter wavelength laser beam sources are increasing in demand.
Such shorter wavelength laser beam sources can improve recording density and photosensitivity and can be applied in the field of optical appliances such as optical memory discs and laser printers.
Thus, second harmonic generating (SHG) devices which can convert the wavelength of the incident laser beam into a 1/2 level have been studied.
A bulk of nonlinear optical single crystal has conventionally been used in such SHG devices utilizing high-output gas lasers as the light sources. However, gas laser is now being replaced mainly by semiconductor laser since there are strong demands for much smaller devices such as optical memory disc apparatus and laser printers, and since the semiconductor laser is less expensive and allows direct light modulation while the gas laser requires an external light modulator. Accordingly, thin film wave-guiding SHG devices are now in demand in order to obtain high conversion efficiency using a low light source output of several mW to several tens of mW.
As the nonlinear optical materials for such thin film wave-guiding SHG devices, those having a waveguide layer of a bulk of lithium niobate single crystal on which Ti or the like is diffused so as to modify refractive index, and those having a waveguide made of a thin film of lithium niobate single crystal formed on a lithium tantalate substrate by high-frequency sputtering process are known. However, it is difficult to obtain a thin film of lithium niobate single crystal having excellent crystallinity in either of these methods, and thus they failed to afford high conversion efficiency.
By the way, the Liquid Phase Epitaxial Method is supposed to be an ideal way of forming a thin film of single crystal having excellent crystallinity.
The Liquid Phase Epitaxial Method for forming a thin film of lithium niobate is described, for example, in:
(1) Applied Physics Letters, Vol. 26, No. 1, January 1975, wherein a thin film of lithium niobate is formed as an optical waveguide on a lithium tantalate substrate by liquid phase epitaxial growth using Li.sub.2 O and V.sub.2 O.sub.5 as a flux;
(2) Japanese Patent Publication No. 9720/1976, wherein a thin film of lithium niobate is formed as an optical waveguide on a lithium tantalate substrate by liquid phase epitaxial growth using Li.sub.2 O and V.sub.2 O.sub.5 as a flux; and
(3) Japanese Patent Publication No. 47160/1981, wherein a solid solution thin film of lithium niobate/lithium tantalate single crystal containing magnesium is formed on a substrate by epitaxial growth using Li.sub.2 O and V.sub.2 O.sub.5 as a flux.
However, the known Liquid Phase Epitaxial Method is neither successful in forming a thin film of lithium niobate single crystal having excellent crystallinity on the lithium tantalate substrate nor can form a film of lithium niobate single crystal having a sufficient film thickness required for producing a SHG device, and thus no practical thin film wave-guiding SHG device is so far known.
The film thickness required for producing a thin film wave-guiding SHG device is such that the effective index of an incident laser beam having a fundamental wavelength of .lambda. can be made consistent with that of the second harmonic with a wavelength of .lambda./2 in order to achieve phase-matching between them. Particularly, when a SHG device for semiconductor laser is produced using a thin film of lithium niobate formed on a lithium tantalate substrate, the thin film of lithium niobate single crystal should have a thickness of not less than 5 .mu.m so as to achieve such consistency between these two effective indexes.
On the other hand, in order to obtain a high-output thin film wave-guiding SHG device, a greater index difference must be secured between the substrate and the thin film waveguide layer, and studies are made for reducing refractive index of the substrate; for example, as disclosed in:
(4) Japanese Patent Publication No. 34722/1985, wherein magnesium oxide and vanadium pentaoxide are simultaneously incorporated to a lithium tantalate single crystal substrate to form a thin film of lithium tantalate single crystal thereon by epitaxial growth method; and
(5) Japanese Patent Publication No. 27681/1988, wherein vanadium pentaoxide is diffused on a lithium tantalate single crystal substrate to form a diffusion layer consisting of vanadium pentaoxide and lithium tantalate with a thickness of 5 to 6 .mu.m and a low refractive index, followed by epitaxial growth of a layer of lithium tantalate single crystal thereon to form a triple-layered optical waveguide.
These methods disclosed all use lithium tantalate as the thin film waveguide layer and are not the technique for forming a thin film of lithium niobate single crystal having excellent optical properties on the lithium tantalate substrate.
In addition to the above methods, the following are also known as disclosed in:
(6) Journal of Crystal Growth, Vol. 54 (1981) pp. 572-576, wherein sodium is incorporated into lithium niobate to form a sodium-containing thin film of lithium niobate single crystal having a thickness of 20 .mu.m on a Y-cut lithium niobate substrate by liquid phase epitaxial growth;
(7) Journal of Crystal Growth, Vol. 84 (1987) pp. 409-412, wherein sodium is incorporated into lithium niobate to form a sodium-containing thin film of lithium niobate single crystal on a Y-cut lithium tantalate substrate by liquid phase epitaxial growth;
(8) U.S. Pat. No. 4,093,781, wherein a strain-free lithium ferrite film is formed on a substrate by liquid phase epitaxial growth by partly substituting lithium with sodium to effect matching of lattice constant of the film with that of the substrate; and
(9) Japanese Provisional Patent Publication No. 142477/1977, wherein crystal grains are allowed to grow slowly and naturally by allowing crystallization to occur very slowly to obtain a strain-free crystal by liquid phase epitaxial growth.
None of the above techniques of (6) to (9) discloses use of the thin film as optical materials.
As described above, there has not so far been disclosed a thin film of lithium niobate single crystal, formed on the lithium tantalate substrate, having excellent optical properties and a sufficient film thickness for providing optical devices including SHG devices.
The present inventors made various studies with a view to solving these problems to notice that a thin film of lithium niobate single crystal having a film thickness sufficient for producing optical devices including SHG devices and also excellent optical properties such as low optical damage (change in the refractive index of a crystal to be caused by irradiation of a high intensity light) and very low propagation loss can practically be formed by subjecting the thin film of lithium niobate single crystal and the lithium tantalate substrate to lattice matching, and they previously accomplished an invention (see Japanese Patent Application No. 247179/1990).
As the result of their further study, they found that if the lithium niobate single crystal has some degree of propagation loss, it can sufficiently be used as an optical device.
On the other hand, according to the conventional liquid phase epitaxial growth method, for example, as described in Applied Physics Letter, Vol. 24, No. 9, 1974, pp. 424-426, the thin film of lithium niobate single crystal formed on a lithium tantalate substrate comes to have an electrooptical constant of merely about 1/3 that of the bulk of lithium niobate.
Thus, the present inventors found that this is because of the difference between the lattice constant of the thin film of lithium niobate single crystal and that of the lithium tantalate substrate and that a thin film of lithium niobate single crystal having a high electrooptical constant which has never been attained can be formed by achieving lattice matching between them, and they accomplished this invention.